1. Field of the Invention
The present invention relates to a counter electrode used in an electrochromic device. More particularly, it relates to a counter electrode in which an optical property in visible region is not substantially changed though reversible change of ions and electrons is caused by controlling its electric field and it also relates to a transmissive electrochromic device using the same.
2. Description of Prior Arts
Heretofore, certain electrochromic devices using a tungsten oxide film have been proposed.
It has been illustrated that color centers are formed by the following equation by biasing the tungsten oxide film in negative EQU WO.sub.3 (colorless)+XM.sup.+ +xe.sup.- .fwdarw.M.sub.x WO.sub.3 (blue color)
wherein M.sup.+ designates a proton, an alkali metal ion or silver ion.
Such electrochromic devices are classified into a liquid type and a solid type. In the former system, the display electrode faces the counter electrode and a liquid electrolyte is filled between them.
In the structures of the electrochromic devices, symmetric and asymmetric devices have been proposed. The symmetric device comprises a transparent substrate (1) having a transparent conductive film (2) and a transparent substrate (4) having a conductive film (5), electrochromic films (3), (6) which are respectively formed on the conductive films (2) or (5) and an electrolyte (7) which is kept between the pair of the substrates and is sealed by sealant (8), as shown in FIG. 1. The asymmetric device comprises an inactive electrode substance such as a metal and carbon as the counter electrode.
Now, the inactive electrode has been considered not to be enough in view of its life. It is usual to employ the symmetric device.
In the symmetric device, the tungsten oxide film is formed on the display electrode and the counter electrode. Accordingly, the tungsten oxide film of the counter electrode is the colored state even though no display is performed. In order to obstruct the coloring state from an observer, a masking material (9) has been required.
Accordingly, the electrochromic device using a counter electrode made of an amorphous tungsten oxide must be a reflective type device.
In the latter device, a display electrode faces a counter electrode and a solid insulating film which injects ions only is located between the electrodes.
A device shown in FIG. 2 comprises a transparent substrate (11) having a tungsten oxide film (13), an ion permeable insulating film (14) made of CaF.sub.2 and a counter electrode (15) of gold film. Since the gold film is used as the counter electrode (15) of the device, its transmission is inferior. Moreover, any reversible process is not be given as the reaction of the counter electrode (15) whereby a gas is generated to shorten its life, disadvantageously.
A device shown in FIG. 3 comprises a transparent substrate (21) having a transparent conductive film (22), a tungsten oxide film (23), an ion permeable insulating film (24) made of RbAg.sub.4 I.sub.5 which transmits silver ions, and a counter electrode (25) of silver film.
In the device, the reaction of Ag.fwdarw.Ag.sup.+ +e is caused on the counter electrode whereby balance of the charges can be maintained. However, RbAg.sub.4 I.sub.5 is not stable. Silver ions are discharged on the display electrode whereby it is disadvantageously deposited in dentrite form. Moreover, a size of silver ion is large whereby a response is disadvantageously slow.
A device shown in FIG. 4 comprises a transparent substrate (31) having a transparent conductive film (32), a tungsten oxide film (33), an ion permeable insulating film (34) of chromia and a counter electrode (35) of gold film.
In the device, protons derived from a small amount of water in the chromia are used as the carrier of charge. Since the gold film is used as the counter film, its transmission is disadvantageously low.